My father passed away a month ago and left us quite a few bikes to deal with one is a 1972 CB125K3 with only 4000 on the clock.I wanted to get it running because it needs to be sold along with all the others but I cannot get the fuel cap off I have sprayed wd40 underneath it as best I can.I know I can replace the cap cheaply but I don't want to damage the tank using too much force.

The 1956 Norton cap is also seized I am not going to attempt to get her running I don't have much knowledge of British bikes.Amazingly after all the years she has been standing nothing but the tank cap is seized.
Does anyone have any ideas what I could use to get the cap off? I thought of getting two pieces of wood and putting bolts through to join them together and clamping that round the cap to get leverage?

Remove the fuel tap and drain any old fuel out.
Leave the fuel tap out to give you a vent hole.
Now try heat on the cap but only in the form of hot water from a kettle sloshed over the fuel cap.
Not a flame obviously.
Then get hold of it with water pump pliers and turn it.
It probably needs a push and turn?
Protect the tank with an old towel or blanket.